Gas turbine engines of the type used for industrial applications may employ combustor systems designed to minimize nitrogen oxide emissions. One such combustor system, disclosed in U.S. Pat. No. 5,481,866, entitled Single Stage Premixed Constant Fuel/Air Ratio Combustor, issued to Mowill on Jan. 9, 1996, is incorporated herein by reference to the extent necessary for a full understanding of such a combustor. The '866 patent discloses a combustor having an externally cooled non-perforated combustor liner that receives all combustion air from a venturi shaped premixer. Excess air that does not enter the combustor through the premixer is ducted so as to externally cool the combustor liner, and eventually re-enters the flowpath downstream of the combustion region through dilution ports. An air valve is used to directly control the amount of air supplied to the premixer so as to minimize nitrous oxide emissions at all power settings. The air valve has the effect of indirectly controlling the amount of air routed to the dilution ports.
A problem occurs when combustors of the type disclosed in the '866 patent are used in conjunction with an engine having a compressor with a relatively high compression ratio. At low engine power settings, the valve used to control air to the premixer is mostly closed forcing most of the compressed air through the dilution ports. Although engine power is reduced, the total volume of air being pumped by the compressor at low power or idle settings remains high, resulting in a substantial increase in dilution airflow at reduced power. However, the dilution ports are necessarily sized to provide adequate backflow margin at the lower flow, higher power settings. Thus at reduced power, higher dilution flow conditions, the dilution ports overly restrict the dilution airflow causing a larger than desired pressure drop across the combustor and a loss of engine efficiency.
One solution has been to provide a separate apparatus for varying the flow area of the dilution ports at different power settings in addition to a valve for controlling air to the premixer. A disadvantage is that such apparatus are typically very complex, adding significantly to the total cost of the combustor system.
Accordingly, a need exists in a low nitrous oxide emissions combustor system for a simplified method of reintroducing excess air not used for combustion back into the flowpath downstream of the combustion zone without the complexity and expense associated with variable area dilution ports.